A wartime graveyard containing the remains of Chinese soldiers who died fighting the Japanese has been destroyed during drainage works, mainland China media has reported.

Photographs of the destroyed graveyard in Nanyang in the central province of Hunan were posted online last week by volunteers, showing the bones of soldiers who died in the second Sino-Japanese war, Cover News reported.

The conflict, which killed millions of Chinese soldiers and civilians, started with the Japanese invasion of China in 1937 and only ended with Japan’s defeat in the second world war.

Nanyang Security Bureau told the newspaper that the site had been destroyed in late June while a 100-metre drainage ditch was being built by Tianxin Properties Limited. The firm could not be reached for comment.

The soldiers’ remains had been buried at the site, which was near to a military hospital. The cemetery was an unofficial one established by locals after the war and had never been given an official designation.

The report said the site had long been neglected and suffered serious damage during the Cultural Revolution and parts of it had been built over in the 1980s.

A mainland law passed earlier this year makes it illegal to “invade, destroy or deface the memorial facilities of heroic martyrs” but it was not immediately clear whether any action would be taken against the company responsible for digging up the site.

However, some Weibo users questioned whether the firm’s actions counted as “insulting” war heroes and one described its activities as “weird and infuriating”.